Observation optical systems used in loupes, binoculars, and terrestrial telescopes, and the like have been generally Keplerian-type (real image-type) optical systems, in which an inverted image formed by an objective system is inverted into an erect image by an erecting system such as prism and the image is observed through an eyepiece system. Since zoom optical systems with a magnification ratio of about 2 times are easily downsized, the zoom type involving zooming the inverted image with a pair of lenses that sandwich the inverted image is generally employed as proposed in Patent Literature 1.
For example, in Example 1 of Patent Literature 1, one of moving groups includes a doublet lens and another moving group includes a doublet lens and a positive lens to correct aberration variation during variation of magnification in the moving groups, and a fixed group includes a single lens. In Example 2 of Patent Literature 1, the group closest to the pupil in the eyepiece system includes a doublet lens to correct axial chromatic aberration.